The present invention relates generally to a pressure relief device for a vessel and more particularly to a resealing pressure device valve for the vacuum vessel of a superconducting magnet, having redundant pressure relief capabilities.
The Collider Detector, Fermilab (CDF) utilizes a superconducting solenoid to produce a horizontal magnetic field used to study particle collisions in the Tevatron. The vacuum vessel which houses the superconducting coils consists of two concentric cylinders. The walls of these concentric cylinders are made as thin as design requirements will permit.
Under normal design conditions, the vacuum vessel is a dewar, that is the space between the two cylinders is a vacuum. Thus, the working pressures that the cylinder walls are designed for, assume a vacuum between the two cylindrical shells.
The maximum allowable working pressure (MAWP) for a cylindrical shell where the pressure is compressing the shell radially inward is much lower than the MAWP, for a cylindrical shell of similar dimensions, where the pressure is pushing the cylindrical shell radially outward.
Since the CDF vacuum vessel is constructed of two concentric cylindrical shells instead of a solid cylindrical tank, it is more sensitive to pressure excursions than most vacuum vessels. As stated above the vacuum vessel walls are designed as thin as possible, under the assumption that the space between the two shells will be a vacuum. Therefore, the inner shell wall is designed assuming that the vacuum will pull the wall radially outward. A loss of vacuum to the vessel, however, may result in a higher pressure in the vessel than the ambient pressure. This higher pressure would compress the inner cylindrical shell radially inward. A pressure difference of as little as 3 psig would result in a failure of the inner shell.
Replacement of the vacuum vessel and any equipment damaged due to such failure would pose a great expense. Thus, the CDF vacuum vessel must have a reliable pressure relief system to protect it from pressure excursions. The pressure relief system must assure that the vacuum vessel will be relieved at about 1 psig (16 psia).
Spring activated pressure relief valves and nonreclosing weight type pressure relief devices are available commercially. A nonreclosing weight type pressure relief device has been built and used at Fermi. The device includes a single solid plate overlying a flange. The topside of the flange has circular grooves in which o-rings are placed to form a seal between the plate and the flange. Similar commercially available relief devices provide means for reclosing the top plate after conditions have returned to normal. These relief devices however do not provide any redundant pressure relief capabilities. If the seal between the plate and the flange were to stick, the vessel would not be relieved at the desired pressure.
Given the low tolerance of the system, a mechanical failure of the relief device would lead to the collapse of the inner cylindrical shell of the vacuum vessel. A pressure relief device need only stick for a few psi over the valve design pressure, to cause a failure of the vacuum vessel. Therefore, a single commercially available relief device would not provide a reliable pressure relief system, since a single commercially available relief device does not provide any redundant capabilities, in the event of device failure.
Additional reliability could be added to the system, using available pressure relief devices, by adding a second safety relief device in the vacuum vessel. This would assure that the vacuum vessel would be relieved at the desired pressure, even if one of the safety relief devices were to stick. Space limitation on the vacuum vessel however, make a dual relief valve system undesirable. Most of the vacuum vessel is surrounded by other components of the detector, limiting the amount of space available. Further, the instrumentation for the CDF requires as much available space as possible. Thus, although an additional relief device would provide redundant safety relief reliability, space limitations would make the system undesirable.
Another pressure relief device available commercially is the rupture disc. The rupture disc comprises a sheet of metal that will break at a given pressure. These devices, however, are not available for the pressures needed in the CDF vacuum vessel. Additionally, these devices have variable tolerance of 10-15 psi. These variable tolerances are well above the failure point of the vacuum vessel making the rupture disc an unacceptable alternate.
Therefore, in view of the above, it is an object of the present invention to provide a pressure relief device which will open at a predetermined pressure.
It is another object of the present invention to provide a pressure relief device with redundant pressure relief capabilities which will open at first predetermined pressure and also at a second, slightly higher, predetermined pressure.
It is another object of the present invention to provide a pressure relief device with redundant pressure relief capabilities which will not require more space than a single pressure relief valve.
It is still another object of the present invention to provide a pressure relief device which will operate in a narrow range of tolerances.
It is still another object of the present invention to provide a pressure relief device with redundant pressure relief capabilities which will reclose after normal conditions have been restored, that is a relief valve.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.